The present invention pertains, in general, to operating diverse types of radio transmitters in shared spectrum in a given geographical area.
There is an ever-increasing demand for radio communications which utilize large segments (or bands) of the electromagnetic spectrum. However, the spectrum is a finite commodity, and there is not enough to serve every projected need. Thus, electromagnetic spectrum is an extremely valuable commodity.
Spectrum is allocated in discrete segments for assigned uses, usually navigation, communication or some combination of the two. The Federal Communications Commission (FCC) closely regulates the spectrum to ensure that transmitters operate within carefully defined limits of frequency, time, and geographical area. Limits on geographical area are often expressed in terms of minimum distances of separation for two transmitters; limits on time of operation are sometimes expressed as time slots.
The FCC also regulates the operation of satellite (space-based) systems which illuminate the United States with transmissions from space. Thus, satellite systems vie for this finite spectrum with terrestrial (ground-based) systems.
Often, FCC spectrum allocations mete out spectrum in bands which become, over time, under-utilized. In the prior art, a number of methods have been developed to maximize the utility of available spectrum.
An historical example illustrates how these developments can occur. The FCC originally allocated L-band, 960-1215 MHz, to a military aircraft navigational system for TACtical Air Navigation (TACAN). As pressures grew to add functions to aircraft, the allocation was changed to permit a second system to share the L-band with TACAN. The second system, Identify Friend or Foe (IFF), was allocated two narrow sub-bands in the L-band, centered at 1030 MHz and 1090 MHz. This method of dividing frequencies in a spectral band is one way to reconcile needs of more than one service.
Still later, a third system was needed and, as a result of an extensive testing effort, it was determined that the L-band could support this third system, the Joint Tactical Information Distribution Service (JTIDS). JTIDS was given a limited allocation to use the L-band spectrum, but only at low power and only at specified distances from TACAN users. JTIDS was also designed to avoid the two IFF sub-bands centered at 1030 MHz and 1090 MHz. JTIDS, a spread spectrum, frequency-hopped communications system, was thus accommodated. By dividing the frequencies up between TACAN and IFF, and then utilizing spatial, spectral and temporal restrictions for JTIDS, the FCC brought about more effecient use of the available L-band spectrum. This example illustrates how, for terrestrial systems users, similar types of users may be accommodated in a given band of spectrum for a given geographical area.
At present, however, other types of users must be considered; namely, those with satellite systems. Satellite transmitters have different problems that cannot be dealt with using narrow sub-bands or low powered signals, for such transmitters may illuminate large areas of the earth with a down linked signal. Furthermore, earth stations transmitting up link signals back to those satellites may be transmitting in nearly horizontal low orbit directions, thus irradiating large portions along the earth's surface between the earth station and the satellite. These satellite and earth station transmitters may interfere with terrestrial communications, in a given common spectral band, unless steps are taken to sort out how the spectrum is utilized.
As an example, it is noted that one system which is currently competing for spectrum is the space-based Teledesic (TM) communications system. The Teledesic system uses 800 MHz of bandwidth in two separate 400 MHz bands (for standard terminal subscribers). The system provides uplink transmissions in the 28 GHz band and downlink transmissions in the 19 GHz band. Teledesic subscribers will have difficulty in using the 28 GHz band, however, unless some accommodation is made for the terrestrial Local Multipoint Distribution Service (LMDS), for the LMDS system also uses the 28 Ghz band.